<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>开关灯案例</title>
  <style>
    /*.bj {*/
    /*  background-color: pink;*/
    /*}*/
  </style>
</head>
<body>
  <button id="btn">开关灯</button>
  <script>
    var btn = document.getElementById('btn')
    var flag = 0
    btn.onclick = function () {
      if (flag === 0) {
        document.body.style.backgroundColor = 'black'
        flag = 1
      } else {
        document.body.style.backgroundColor = '#fff'
        flag = 0
      }
    }
    // btn.addEventListener('click', function () {
    //   document.body.classList.toggle('bj')
    // })
  </script>
</body>
</html>